This invention relates to lead frames and semiconductor devices formed with a lead frame, and to a method for manufacturing a semiconductor device with a lead frame.
Many semiconductor devices having a number of lead terminals spaced at extremely small intervals must be mounted on a lead frame having leads which are spaced at correspondingly small intervals and tie bars which join the leads prior to assembly and must be cut with a high degree of accuracy. The step by which the semiconductor device is sealed in resin, which occurs prior to cutting of the tie bars, tends to displace the leads, making it difficult to cut the tie bars accurately.
One conventional process for manufacturing a semiconductor device is shown in FIGS. 3 and 4. In that process, a semiconductor chip (not visible in the drawings) including a semiconductor circuit is placed on the die pad of a lead frame 1, and the electrodes of the semiconductor chip are electrically connected to closely-spaced leads 2 arranged around the die pad with metal wires such as gold wires. Thereafter, the semiconductor chip on the die pad, the metal wires, and parts of the leads 2 are sealingly covered with resin 11, and then the closely-spaced leads 2 are separated from the lead frame 1 and from each other by a cutting operation.
As shown in FIG. 3, the leads 2 are spaced at predetermined close intervals in the lead frame and they are joined to one another by a series of tie bars 3 and coupling bars 4, and are positioned in the lead frame by narrow supports 5 connected to the side rails 6 of the lead frame. The tie bars 3 also have another function in that they serve as stoppers to stop the flow of resin when the semiconductor chip is covered with resin as described above. During the coating of the semiconductor chip with resin, a problem arises because the leads 2 tend to be deformed, for instance, by contraction of the mold. To eliminate this problem, the narrow supports 5 provide a buffer arrangement in the lead frame to allow the leads to move with respect to the frame during contraction of the resin. Alternatively, narrow supports of Rahmen (rigid-frame) structure or bellows structure may be used. In separating the leads from the lead frame 1 after the chip is covered with resin, the lead frame is positioned by using positioning holes 9 formed in the side rails. With the lead frame thus held in position, a punch 13 is used to remove the tie bars 3 as shown in FIG. 4 and the coupling bars are similarly cut off.
As described above, when the semiconductor device is sealingly covered with resin, a contracting force acts on the leads as the resin sets, so that the leads are moved in the frame. In this operation, the narrow supports are deformed, thus absorbing the contracting force. Hence, after the semiconductor chip has been sealingly covered with resin, the leads may be shifted in position with respect to the side rails of the lead frame. In cutting the tie bars, however, the positioning holes formed in the side rails are employed as a positioning reference and, consequently, the punch cannot be accurately positioned with respect to the leads. Because of such inaccurate positioning during cutting, the resultant leads are unsatisfactory in both electrical and mechanical characteristics.
Furthermore, there has been a recent tendency for ICs or LSIs to be smaller in size and to have a larger number of functions, requiring an increase in the number of leads. As a result, the leads are even narrower, as is the spacing between the leads. In these devices, so-called "fine-pitch leads" are employed, having, for instance, a lead width of 0.1 mm, a pitch of 0.3 mm, and a lead separation of 0.2 mm. Accordingly, the fact that the leads are shifted in position during sealing as described above presents a serious problem and further impedes the manufacture of semiconductor devices having "ultra-fine-pitch" leads which are spaced at even smaller intervals.
Japanese Unexamined Published Application No. 2-17637 discloses a lead frame of the type shown in FIG. 5. That lead frame contains a positioning hole 16 in a reference part 15 which is connected by a support 14 to the die pad of the lead frame instead of to the side rail 6, and is also connected to the tie bars 3. The reference part 15 is thus supported only by the leads and the die pad which are sealingly covered with resin, and therefore it is relatively free from contraction of the resin. Accordingly, its relative position with respect to the leads can be maintained substantially unchanged during the sealing operation. On the other hand, the reference part 15 is located away from the leads, and it is supported only by the tie bars 3 and the support 14, thus being free at the opposite end. As a result, the reference part 15 is liable to be deformed by even a weak external force during manufacture. That is, it is liable to be shifted in position with respect to the leads before the device is sealingly covered with resin.